The present invention relates to processes and apparatus for the drying of moist particulate granulate material and classifying the material to provide a substantially dry particulate granulate product. The particular product dried by this process is a ground specular hematite used as a conditioning or weighting agent for oil field drilling muds. This material is basically an iron ore that is ground to a fine almost powder-like consistency then transported in a water slurry from the grinding site to storage that is usually outdoors. In arid climates it is necessary to dampen the outdoor storage piles of material in order to prevent it from blowing.
In this somewhat moist state this material has a tendency to stick on surfaces such as the sidewalls etc. in bulk handling pneumatic conveying equipment. Therefore, in order to be properly fluidized by pneumatic conveying equipment it is necessary to dry this material to a low moisture content in order for it to be handled in this type of conveying equipment. In the present drilling fluid industry the equipment utilized to transport large quantities of bulk materials such as the ground specular hematite material mentioned above is pneumatic conveying systems that fluidize the particulate material for transport through conduits into reservoirs, containers, hoppers, etc. that are adapted for handling this type of particulate material as is well known in the art. Because of the need to be handled in this manner and transported by trucks from the processing plant to a well site or remote oil field storage facility the material must be adapted for this convenient and traditional handling procedure. Because of the particular characteristics of this material unless it is very dry it will stick to the sides of the conduits and containers thus making transportation substantially more difficult as well as requiring substantial cleaning and maintenance of the equipment.
In the prior art processes the drying of slurries and very moist particulate granulate material has been done in cement manufacturing processes by using a rotary kiln to evaporate the moisture as well as other apparatus to reduce the particle size. One example of this is in U.S. Pat. No. 2,290,068 to L. S. Peterson wherein a raw slurry is mixed with the dried product and this mixture dried in a rotary kiln. This patent represents one general technique for drying this character of material wherein a dried material is mixed with a raw moist material and the combination dried together.
Another concept in drying a slurry or moist product is represented in U.S. Pat. No. 3,592,395 to R. M. Lockwood wherein a fluidized bed dryer is provided to stir and dehydrate the product by the motion of air through the product and in conjunction with a rotary agitator to stir the product.
Other drying apparatus for this type of material is shown in U.S. Pat. No. 4,070,765 to S. Hovmand et al wherein a pneumatic conveyor dryer is used in the drying process. This apparatus also includes recycling a portion of the dried material as well as classifying the material to remove oversized particles as well as disintegration of the oversized particles.
The deficiencies in the prior art equipment and processes for drying the specular hematite material lie in the need for continuous drying and dust removal aspects as well as classifying the material to remove a typically small quantity of oversized particles. While the above noted processes could possibly be used to dry the moist specular hematite they would be inefficient in that it has been found that recycling of the dried material into the raw product stream is unnecessary and classification of the material is typically simple due to the small quantity of oversized particles.